1. Field of the Invention
The invention relates generally to a system and a method for inspecting fasteners, such as screws.
2. Description of the Prior Art
In assembly operations where fasteners such as threaded screws are automatically installed by a machine, defective fasteners cause damage to the product being assembled, resulting in costly repairs. Thus, it is critical for quality control to inspect fasteners for surface flaws prior to use. In the past, fasteners have been manually and visually inspected by workers. However, threaded fasteners are normally made at a high volume, e.g. several thousand fasteners per hour. Visual inspection to sort out defected fasteners from such high volume production has been known to be very time consuming and lacking in adequate precision. Sampling programs were implemented for monitoring the quality of high volume production of threaded fasteners. However, recent zero defect demands for threaded fasteners by the end users have resulted in the requirement that the fastener suppliers inspect 100 percent of the threaded fasteners prior to shipping to the end users. Statistical sampling is no longer an acceptable testing technique.
There is an increasing need for an evaluation of all critical dimensional criteria of individual threaded fasteners by the suppliers to ensure that defective threaded fasteners are removed before shipment.
As an attempt to provide 100 percent inspection of threaded fasteners, several non-contact inspection systems have been developed for automatically performing various inspections.
U.S. Pat. No. 4,457,622, issued to Kato et al., discloses a screw inspection device, which includes a screw transfer mechanism, a sensor for performing predetermined measurements of the screw during the transfer, a comparator for generating acceptance/non-acceptance signals, and a sorter for classifying the screws into defective and nondefective screws.
U.S. Pat. No. 4,598,998, issued to Kamei et al., discloses an inspection system in which light is projected onto the threaded surface of a fastener, and the surface flaws are detected based on the variation of the intensity of the reflected light.
U.S. Pat. No. 4,823,396, issued to Thompson, discloses an automated inspection device, which includes a camera for producing a video image of a fastener and a computer for comparing the actual dimensions of the fastener with the desired dimensions.
U.S. Pat. No. 5,823,356, issued to Goodrich et al., discloses an inspection device in which threaded fasteners can be continuously supplied and moved into a test station where the threaded profiles of the fasteners are functionally tested. A plurality of sensors are disposed along the transfer path through the inspection device to perform predetermined measurements of the specified dimensional characteristics of the fasteners. A sorting device is disposed close to the end of the transfer path to separate defective fasteners from nondefective ones.
Although conventional non-contact inspection systems are very useful, they all have certain limitations. One limitation is that the types of flaws detected are limited. In order to increase the number of geometry features to be inspected, a complex set-up of sensors and hardware would be required. Another limitation is that the conventional inspection systems are not easily adapted for different fastener sizes and types. In addition, the conventional systems are not capable of inspecting and sorting high volume of small fasteners at high speed.